MEXICO CITY (AP) — A water-heating system may have leaked natural gas into a tunnel beneath the headquarters of Mexico’s national oil company for more than seven months before it was accidentally detonated by a maintenance crew’s improvised lighting system, officials said Tuesday, adding fresh detail to the narrative of the petroleum giant’s worst disaster in a decade.

Mexico’s attorney general said Monday night that a buildup of an unspecified gas was responsible for the explosion that collapsed three floors of the administrative building in Petroleos Mexicanos’ Mexico City headquarters complex, killing 37 people. He indicated the gas could have been either natural gas, which is used to fuel water boilers, or methane, a product of decomposition found in sewers and landfills.

Assistant Attorney General Alfredo Castillo told reporters Tuesday morning that the likeliest source of the gas was a tunnel that ran beneath the devastated building and carried hot water from a natural-gas heating plant to the 54-story central tower of the complex. He said that explanation appeared likely because the blast blew off manhole covers providing access to the tunnel some distance from the affected building. He added, however, that a methane gas buildup had not been entirely discounted.

Attorney General Jesus Murillo Karam said an independent contractor had told investigators that he was working with a crew of three men performing maintenance in the basement of building B2 on Thursday afternoon. The contractor said the basement wasn’t lit, so his crew had rigged illumination by attaching a crude electric cable to a power source in the ceiling.

The contractor told investigators that seconds after he moved to a higher floor, “he heard a strong, sharp whistling through the corridor, coming from the area of the foundation pilings that was being worked on, and then right away he felt a strong explosion that threw him against the wall,” Karam said.

The three men were found dead in the lower basement with burn marks, one with a fragment of cable stuck to his body.

Castillo said the maintenance supervisor reported that his crew had not been in the lower basement to inspect the foundations in seven or eight months. It was not immediately clear if Pemex, which is responsible for inspecting its own buildings, required more regular maintenance.

Murillo said investigators were still reviewing records of building inspections to determine why Pemex had not discovered the gas accumulation.

After days of speculation that the building had been bombed, Murillo said Mexican, Spanish, U.S. and British investigators looking into the petroleum giant’s worst disaster in more than a decade found no evidence of explosives. With the exception of three victims, none of those killed had the burn marks or damaged ear drums that are typical evidence of a bombing, he said. Nor was there any sign of a crater or fracturing of the building’s steel beams, also common signs of the detonation of an explosive device.

The victims’ bodies were found largely intact, and dismemberment is normally found after the detonation of explosives, he said.

Murillo described a “diffuse” blast that moved slowly and horizontally, typical of the detonation of a cloud of gas, rather than an explosion that would have emanated from a relatively compact source like a bomb.

This explosion, at its peak, generated an effect on the structures of the floors of the building, first pushing them up and then causing them to fall, and that was the primary cause of deaths in the building,” he said.

The announcement late Monday ended days of a near-total lack of information about the potential cause of the incident. The sparse information spawned a torrent of complaints about government secrecy and speculation about the cause of the blast, most focusing on the possibility that it had been set intentionally.

The suspicions of foul play became so intense that Murillo insisted on displaying photos of a backpack found in the rubble to prove that it contained makeup, and not a suspicious, potentially explosive device as reported by some Mexican media earlier in the day.

Murillo said there is not yet any evidence of criminal wrongdoing in the disaster, but the possibility of criminal charges remained open.

Some observers compared the lack of openness by the Institutional Revolutionary Party-led government to the party’s secretiveness during more than seven decades of autocratic rule. The party returned to power in December after losing the Mexican presidency 12 years earlier.

The blast also generated debate about the state of Pemex, a vital source of government revenue that is suffering from decades of underinvestment and has been hit by a recent series of accidents.

The disaster was a major setback to a safety record that had been improving following a series of incidents in the 1980s and 1990s, according to company figures. The number of accidents per million hours worked dropped by more than half, from 1.06 in 2005 to 0.42 in 2010. That is in line with the international average of about 0.43 per million, according to the U.K.-based International Association of Oil and Gas Producers, which does not independently verify company numbers.

But Pemex acknowledged in a report that starting in late 2011, a series of smaller blasts and fires, mainly at refineries and petrochemical plants, had “seriously impacted” its safety rate. It said the rate of injuries per million hours had risen to 0.54.

Mexican President Enrique Pena Nieto has pledged to open the oil behemoth to more private and foreign investment, setting off warnings among leftists about the privatization of an enterprise seen as one of the pillars of the Mexican state. Pena Nieto has provided few details of the reform he will propose but denies any plan to privatize Pemex.

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